Method of making porous inorganic particle filled polyimide foam insulation products

ABSTRACT

A method of making thermal insulating products comprising porous, lightweight, high temperature resistant inorganic particles in a polyimide foam matrix and the product thereof. A polyimide precursor powdered is mixed with about equal weight of flake-like porous inorganic particles to substantially uniformly coat the particles with powder. The mixture is placed in a mold and compressed slightly. The assembly is heated to the foaming temperature of the polyimide precursor for a period suitable to produce uniform foaming. Then the temperature is raised to the curing and crosslinking temperature of the precursor for a time period sufficient to produce complete cure. A high temperature and flame resistant, light weight, highly insulating product results. If desired, protective sheets of material may be bonded to selected product surfaces during or after the molding operation.

This is a division of co-pending application Ser. No. 312,490, filed on2/21/89now U.S. Pat. No. 4,865,784.

BACKGROUND OF THE INVENTION

This invention relates in general to thermal insulating products and,more specifically, to high temperature flame resistant thermalinsulation using a combination of polyimide foam and porous, lightweightinorganic particles in insulating products.

Lightweight, porous inorganic materials such as pumice, expandedvermiculite and "popped" perlite have long been used in thermalinsulation, lightweight concrete, as packing material, etc. Pumice is ahighly porous igneous rock, primarily an aluminum silicate. Perlite is aform of glassy rock, similar to obsidian, which when heated to itssoftening temperature rapidly expands or pops to form a fluffylightweight material similar to pumice. Vermiculite is a mineral of themica group, a hydrated magnesium-aluminum-silicte, having the ability toexpand 6 to 20 times in volume when heated above 1400° F. When used asthermal insulation, these materials are ordinarily used as looseparticles to fill hollow walls or the like, or are incorporated ininorganic binders such as concrete. This insulation is inexpensive anfairly effective, but is not well adapted to complex light weightinsulation shapes, such as pipe insulation;. Further, the inorganicbinders have poor insulating qualities and are heavy.

Organic binders, such as epoxies, have been used in some case to reducethe weight and improve the formability of porous inorganic particlebased insulating materials;. However, these organic binders tend tolimit the temperatures at which the insulating materials can be used toless than 300° F. due to binder degradation. Also, the insulatingqualities of the binder itself tends to be low.

In low temperature applications, with complex insulation shapes, foamedorganic resin insulation has been widely used. Typical of these areexpanded cellular polystyrene as described by Charpentier in U.S. Pat.No. 3,863,908, polyurethane foams as described by Willy in U.S. Pat. No.3,998,884 and phenolics as descried by Bruning et al in U.S. Pat. No.3,883,010. While these materials often have excellent insulatingcharacteristics and are inexpensive to manufacture, they often have lowstrength and low impact resistance and cannot be used at temperaturesabove 300° F. These organic materials degrade at higher temperatures,may burn, and often emit toxic gases at high temperatures or whenexposed to a direct flame.

Recently, a number of polyimide foam insulating materials have beendeveloped, such as those described by Long et al in U.S. Pat. No.4,621,015 and Gagliani et al in U.S. Pat. Nos. 4,506,038 and 4,426,463.These polyimide materials have much greater resistance to hightemperatures, resist burning and degradation when exposed to directflames and do not emit toxic gases at high temperatures.

Additives, such as fibers, talc and microballoons may be added to thefoam material, primarily to improve strength. Such polyimide materials,however, tend to be expensive, require high temperatures for foaming andcuring and tend to have lower melting points than vermiculite whichmelts at a temperature in excess of 2000° F.

Thus, there is a continuing need for improved thermal insulatingmaterials which combine ease of forming and high temperature resistancewith low material and manufacturing costs.

SUMMARY OF THE INVENTION

The above-noted problems, and others, are overcome by the method andproduct of this invention in which particles of porous, lightweight,high temperature resistant, inorganic particles are mixed with apolyimide precursor powder until the particles are coated with thepowder, the mixture is placed in a mold, slightly compressed, heated tothe foaming temperature of the precursor for a suitable period, thenheated to a suitable higher temperature at which the foam cures andcrosslinks.

The resulting insulation product, which may typically be in the form ofa tube or half-tubes adapted to surround pipes, is strong and wellbonded, highly insulating due to the fact that both particles and binderare porous, resistant to high temperatures and direct flame and low inmaterial and forming costs.

If desired, surface sheets or layers may be bonded to any surface of theinsulating product for increased impact resistance, lower emanations orthe like. Typically, such a surface sheet may be bonded to the interioror exterior surface of a tubular pipe insulation sleeve. Any suitablesheet material may be used. Typical sheet materials include thin metalfoil such as 2 mil thick stainless steel, aluminum or other metal, afabric or matt sheet such as glass fiber cloth or the like. The sheetmay be bonded with a suitable adhesive after molding of the product.Preferably, a polyimide adhesive is used to maintain the hightemperature resistance of the assembly. Alternately, the sheet may beplaced in the mold prior to introduction of the powder-particle mixture,to be bonded simultaneously with the molding operation.

BRIEF DESCRIPTION OF THE DRAWING

Details of the invention, and of one preferred embodiment thereof, willbe further understood upon reference to the drawing, wherein:

FIG. 1 is a schematic flow diagram of the basic method of thisinvention;

FIG. 2 is a schematic exploded perspective view of a tubular insulationproduct made by the method of this invention.

FIG. 3 is a perspective view of a tubular insulation product made by themethod of this invention.

DETAILS OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is seen a flow diagram of the basic stepsin the method of this invention.

As indicated in block 10, a suitable polyimide precursor is selected.Any suitable foamable polyimide precursor which can be caused to foam byheating to a first elevated temperature for a suitable period, and thenfully polymerized and crosslinked by heating to a second, higher,temperature for a suitable period may be used. Typical foamablepolyimide precursors are described by Gagliani et al in U.S. Pat. Nos.4,506,038, 4,394,464 and 4,426,463 by Shulman et al in U.S. Pat. No.4,647,597 Long et al in U.S. Pat. No. 4,518,717. We prefer a polyimideprecurser powder material available from the Monsanto Corporation underthe Skybond RI 7271 trademark because it gives a strong product and iscomposed of relatively benign constituents.

The polyimide precursor should be dry and non-tacky and may have anysuitable particle size. For best results, it is preferred that theparticles have average diameters in the 1/16 to 1/4 inch range.

The polyimide precursor powder next is mixed with a suitable quantity ofporous, lightweight, high temperature resistant inorganic particles asindicated, in block 20. Any suitable particle material may be used.Typical such particles include pumice, expanded vermiculite, expandedperlite and mixtures thereof. Vermiculite is preferred because of itslow weight, and excellent insulating quality.

While the particles may have any suitable size and shape, we preferflake-like particles having a greatest dimension of from about 1/4 to3/8 inch and a least dimension of from about 1/32 to 1/16 inch becausethe larger flakes have a lower surface-to-volume ratio, allowing lesspolyimide to be used in coating the flakes. Vermiculite flakes withinthose size ranges are available from W.R. Grace Co, ConstructionProducts Division, Cambridge, Mass. as product grade 1 or 2. For bestresults, all small particles and dust should be sifted out prior to useof the particles in this method.

Before mixing with the polyimide precursor, the inorganic particles arepreferably carefully dried (typically at a temperature of about 125° to300° F. for a suitable period) to eliminate sticking or lumping of themixture as the polyimide precursor and inorganic particles are mixed.

The particles and precursor may be mixed using any suitable mixingequipment. I have found that excellent mixing and uniform coating of theinorganic particles with the precursor powder can be achieved bytumbling them together in a conventional drum tumbler at about 5 to 50rpm for about 10 to 15 minutes.

The mixture is then poured into a suitable mold, as indicated by block30. A preferred mold arrangement is shown in FIGS. 2 and 3. Of course,the mold configuration may be changed to produce insulating products ofwidely varying shapes.

The mold is coated with any suitable mold release material. Typically, aspray coating may be used or the mold may have a surface layer ofpermanent mold release material such as the fluorocarbon materialsavailable under the Teflon trademark from the E.I duPont de Nemours &Co.

Once the mold is filled it is preferably tamped or vibrated slightly toassure uniform filling and settling or the material. When the mold isclosed it is preferred that the contents be slightly compressed,typically about 2-5 vol% to assure intimate, tight contact between theparticles. As is discussed in conjunction with the description of FIGS.2 and 3, below, the mold can be made to provide this compression as itis closed.

The mixture is then heated to foam the polyimide precursor, as indicatedin block 40. The precise optimum temperature and time will depend uponthe specific polyimide precursor selected. The temperature should besufficiently high to assure rapid and complete foaming, but not so highas to significantly polymerize the precursor. Typically, temperatures offrom about 275° F. to 400° F. for periods of about 10 to 40 minutes givegood results. The foamed precursor fills all of the interstices amongthe inorganic particles, bonding them into a unitary mass whileproviding added insulation values from the small, closed, cell nature ofthe foam.

Next, the temperature is increased to the curing and crosslinkingtemperature of the polyimide precursor, as indicated in block 50. Whilethe optimum temperature and time will depend upon the precursorselected, generally curing at from about 400° to 650° F. or from about 1hour or more is effective.

Finally, the well-bonded, unitary insulation product is removed from themold as indicated in block 60. As mentioned above and described furtherin some of the following examples, surface sheets or layers may beapplied to the product after removal from the mold, if desired. Also,such sheets can be placed against one or more interior walls of the moldprior to introduction of the mixture thereinto, so as to bond to theproduct as the foam is expanded in the step of block 40.

FIG. 2 presents a schematic exploded perspective view of one preferredmold assembly suitable for use in this method.

The mold consists of split outer sleeve 70 having a pair of outwardlyextending flanges 72 adjacent to the split 74. A core 76 which may be arolled metal sheet or seamless tubing is adapted to be centered withinsleeve 70. Upper and lower disks, 78 and 80, respectively, fit withinsleeve 70 and have openings 82 for receiving core 76. Bolts 84 (only oneshown, for clarity) fit through holes 86 in flanges 72 to draw themtogether as nuts 88 are tightened on bolts 84. The mold may be made fromany suitable material, such as sheet metal, ceramics, plastics, fiberreinforced plastics or combinations thereof. All interior surfaces arecoated with a mold release agent. Of course, the mold may have a varietyof configurations, depending upon the shape of the insulation product tobe produced. For example, more than one set of flanges 72 may beprovided, spaced around the tube. Multiple flange sets would assist inuniformly compressing the molding material and would be convenient forvery large tubes. Also, the mold could have other shapes, such ashalf-tubes.

In use, lower disk 80 is placed just within sleeve 70 on a flat surface,core 76 is placed within sleeve 70 with the lower end snugly fitting inopening 82 in disk 80. The mixture of polyimide precursor and inorganicparticles is poured into sleeve 70 to the desired level. For bestresults, the assembly is vibrated slightly or the surface of thematerial is tamped lightly to assure proper settling and filling of themold. Upper disk 78 is inserted in sleeve 70 with hold 82 surroundingcore 76 and pressed lightly against the surface of the filling material.

Bolts and nuts 84, 88 are tightened, bringing flanges 72 tightlytogether. Those flanges may be slightly spaced, so that this tighteningprovides a slight compression of the mixture in the mold. Also,tightening of such slightly spaced flanges will assure tight contactbetween sleeve 70 and disks 78 and 80. However, precise sealing of theflanges and the interfaces between disks 78 and 80 against sleeve 70 andcore 76 is not absolutely necessary. Any slight amount of flash extrudedinto these interfaces during filling, foaming and curing of thepolyimide can be easily trimmed off after processing. The filledassembly is then placed in a suitable oven and heated to foam, thencure, the polyimide precursor, as detailed above.

Upon completion of cure, bolts 84 are released and the foamed product 90as seen in FIG. 3 is removed. Any flash along product edges is trimmedaway. The product is strong, unitary and has excellent insulation andhigh temperature resistance properties. The product consists ofinorganic particles 92 bonded securely by closed cell polyimide foam 94.One advantage of the process is that no shrinkage of the finished parthas been noticed. The inner core 76 is easily removable from thefinished part, and the finished part exactly fits the mold with nonoticeable shrinkage. This is a great advantage in making precisionparts of this material. Tooling design is greatly simplified.

There is an advantage in adding the polyimide powder to warmvermiculite, which is from 100° F. in temperature. The powder adheresmuch more easily to the surface of the vermiculite flakes.

Details of several preferred embodiments of the method of this inventionare provided in the following examples. All parts and percentages are byweight, unless otherwise indicated.

EXAMPLE I

A quantity of Grade 2 vermiculate flakes from the W. R. Grace Company issifted to remove fines and powder, then 400 grams is placed in atumbling drum. The flakes are about the size of dried Navy beans, with alength of about 1/8 inch an a thickness of about 1/16 inch. About 400grams of dry, finely powdered polyimide precursor, available from theMonsanto Corporation under the Skybond R17271 trademark, is added to thedrum. The drum, which has an internal diameter of about 16 inches withsix spaced internal 2 inch ribs, is rotated for about 30 minutes atabout 15 rpm, to assure uniform dusting of the vermiculite flakes withthe polyimide precursor powder. A sheet metal mold of the sort shown inFIG. 2 is prepared using stainless steal with a coating of Teflonfluorocarbon on the inside. The mold has a length of about 16 inches, anoutside diameter of bout 53/8 inches and a core diameter of bout 27/8inches. The vermiculite/polyimide precursor mixture is poured into themold and vibrated for a few minutes to assure uniform filling andsettling of the flakes. The upper disk is emplaced and pressed downlightly. The mold is compressed about 4 vol% as the flange bolts aretightened. The mold assembly is placed in a preheated 375° F. oven forabout 30 minutes during which time the polyimide precursor foams butdoes not significantly cure. The temperature is then increased to about500° F. for about 120 minutes, during which time the polyimide foamcures. The mold is cooled to about room temperature and the product isremoved. Little, if any, shrinkage from the mold dimensions hasoccurred. The apparent density of the product is about 9 pounds percubic foot.

EXAMPLE II

The tubular composite product produced in Example I is placed on achemical stand above a Bunsen burner. A thermocouple is placed on theunderside of the product, adjacent to the burner. The burner is turnedon and adjusted to maintain the underside of the product at about 1800°F. for about 30 minutes. No visible smoke issuing from the product isobserved. Examination of the product after removal from the flame showsthat surface scorching occurs only to a depth of about 1/4 inch. Theflakes of vermiculate remain in place and have not dropped off evenwhere scorching is most extreme. The product is thus seen to have theability to maintain structural integrity and insulation ability despiteexposure to high temperatures and flame for extended periods.

EXAMPLE III

A tubular product is prepared as described in Example I. The outside ofthe product is coated with a light coating of about 50 grams of theSkybond R17271 precursor dissolved in about 100 ml ethyl alcohol. Thecoated surface after drying is then wrapped with Vertex 7781 treatedglass cloth from AMATEX Corporation, Norristown, Pa. This assembly isreturned to the mold assembly and put through the same oven heatingcycle as detailed in Example I. Upon removal from the mold, the cloth isfound to adhere very tightly to the outside of the insulation. The clothfacing protects against impact damage and serves as an added flameshield during fires.

EXAMPLE IV

The method of Example I is repeated, with the following changes. Priorto filling the mold with the mixture, a sheet of Vertex 7781 cloth iscoated on one side with the precursor/alcohol solution described inExample III. The cloth after drying is placed against the interiorsurface of the outer sleeve, having sufficient tack to remain in place.The mold is filled and processing is continued as described in ExampleI. The product is found to have the surface sheet well bonded to theouter surface of the tubular insulation product.

EXAMPLE V

A tubular insulation product is prepared as described in Example I. Theinside of the tube is coated with the precursor/alcohol solutiondescribed in Example II. A sheet of 2 mil thick type 321 brightreflective stainless steel foil is rolled up into a cylinder which fitswithin the inside of the tube. The tube is reinstalled in the mold, withthe core holding the foil in place. The assembly is placed in an ovenheated to about 550° F. for about 100 minutes. Upon removal, the foil isfound to be tightly adhered to the inside of the insulation tube. Thebright reflective foil provides protection of the internal surfaceduring installation on pipes to be protected, and acts as a radiationshield, thereby improving the overall insulating effectiveness.Alternatively an air gap is provided between the insulation and the tubeor pipe to be protected. Air gaps of from 1/8 to 1/4-inches giveconsiderable added protection during fires.

EXAMPLE VI

The process of Example I is repeated, except that a single layer sleeveof aluminum foil is wrapped around the core prior to installation in themold assembly. The mold is filled and processed as described in ExampleI. Upon removal from the mold, the aluminum foil is found to be securelyand uniformly bonded to the inside of the insulation tube.

EXAMPLE VII

About 500 grams of expanded perlite having a greatest average dimensionof about 1/4 inch and least average dimension of about 1/16 inch issifted to remove all dust and fines, then is heated at about 200° F. forabout 40 minutes to remove all moisture. The perlite while still warm isthen placed in a tumbling drum with about 400 grams of a dry, finelypowdered polyimide precursor prepared as described by Gagliani et al inU.S. Pat. No. 4,394,464 and ground to a fine powder. The tumbler isoperated for about 40 minutes to assure complete dusting of powder overthe perlite. A mold having the general configuration of that shown inFIG. 2 is prepared. Length is about 10 inches, diameter is about 6inches and the core diameter is about 4 inches. The mixture is pouredinto the mold to the desired level and the surface is tamped lightly tassure uniform filling. The upper disk is emplaced and the flanges aretightened. The mold assembly is placed in a pre-heated oven at about350° F. for about 2 hours, then at about 550° F. for about 2 hours.After cooling to room temperature, the mold assembly is opened. A wellbonded unitary tubular insulation product results. No shrinkage of thepart from the tooling can be observed.

While certain specific conditions, components and proportions have beenspecified in the above description of preferred embodiments, these canbe varied, where suitable, with similar results. For example, othermaterials could be added to the polyimide precursor such as colorants orinfra-red absorbers and additional sheets or structures could be bondedto the completed insulation product.

Other ramifications, applications and modifications of this inventionwill occur to those skilled in the art upon reading this disclosure.Those are intended to be included within the scope of this invention asdefined in the appended claims.

I claim:
 1. The method of making shaped thermal insulation productswhich comprises the steps of:providing a quantity of foamable polyimideprecursor powder; mixing therewith from about 50 to 150 wt% porous,lightweight, high temperature resistant inorganic particles, based onpower weight, until said particles are substantially uniformly coatedwith said powder; said inorganic particles being flake-like, having aleast dimension of from about 1/32 to 1/16 inch and greatest dimensionof about 1/4 to 3/8 inch; pouring said mixture into an open mold;compressing the filled mold contents by about 2 to 5%, closing the mold;heating said filled mold to the foaming temperature of the polyimideprecursor power for a period sufficient to allow substantially completespontaneous foaming to occur; increasing the temperature of the filledmold to a temperature sufficient to cure said precursor to a polyimide;removing the resulting bonded insulation product from the mold; andcoating at least one surface of the insulation product with a solutionof a polyimide precursor, applying a sheet material to the coatedsurface and heating the resulting assembly to cure the polyimideprecursor.
 2. The method according to claim 1 wherein said sheetmaterial is selected from the group consisting of metal foils, fibrousfabrics, fibrous matts and combinations thereof.
 3. The method accordingto claim 1 further including the step of placing a sheet materialagainst at least one interior wall of said mold prior to theintroduction of said mixture thereinto, whereby said sheet materialbonds to the insulation product during the heating steps.
 4. The methodaccording to claim 3 wherein said sheet material is selected from thegroup consisting of metal foils, fibrous fabrics, fibrous matts andcombinations thereof.